Foursquare is currently winning in the check-in services space but I believe it is still anybody's game because there is still a lot of work to do.
Even though I "know" people that work at Gowalla and their sense of design is practicallyunparalleledin the check-in service space - Foursquare simply works better and that is why I use Foursquare instead of Gowalla.
"I’ll be honest, it’s been a while since I’ve used Gowalla (a location-based checkin service you use on your phone). I’ve found that in most of the cities I visit Foursquare has more users, more tips, is faster, easier to check in, etc." - Robert Scoble
This has been my experience as well. Check-in services need to be very, very fast and valuable in order for the mass market to use them. Typically check-in services aren't social streams (that you check many times per day) they are utility apps to help you track locations that you visit, the current location of your friends, and - most of all - information about where you are from other people that have been there. When this entire process isn't very easy and very fast it becomes a hassle to use the services and so I quickly give up on using them.
Foursquare, while far from perfect, is simply lightyears faster than Gowalla currently is. Checking into a location is quick and easy. For the most part, everywhere that I'd like to check-in is already in Foursquare. On Gowalla I had to add nearly every location I visited. This task is tedious on Gowalla. Here is one gripe, as an example: When you search for a location using Gowalla's iPhone application and it isn't in their database (which happened for 85% of my check-ins even in places like Atlantic City, NJ) - you can't tap an "Add location" button. You have to back out of the location search screen and go back into another screen to find the "Add location" button. This became so annoying that I downloaded the Foursquare application and haven't looked back since.
I have had high hopes for check-in services ever since my days using Brightkite (man I miss those days). Foursquare has legs (and $50M in the bank). I hope these services continue to improve but they'll need to innovate very, very quick in order for the mass market to adopt them longterm.